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Runtime Context
Describe runtime context supplied while a bound loop runtime runs.
Runtime context is a volatile per-turn provider, not a second system prompt stored in the session definition:
type RuntimeContextProvider interface {
Blocks(context.Context) []content.Block
}
func WithRuntimeContext(provider RuntimeContextProvider) OptionThe provider commonly supplies date, current directory, or Git-state blocks. Harness calls it at the turn tail and appends returned blocks to that request’s context. A nil or empty slice appends nothing.
Per-turn behavior
The callback receives the turn context and must be cheap and non-fatal. The contract deliberately has no error return: an implementation failure should degrade to fewer blocks, never fail the turn or cause a retry storm. The provider must not mutate a slice it returned after the call; the runtime treats the blocks as input for that request.
type gitContext struct{}
func (gitContext) Blocks(ctx context.Context) []content.Block {
// Read-only, bounded collection. Return nil if the context is cancelled or
// the optional source is unavailable.
return []content.Block{&content.TextBlock{Text: "cwd: /work"}}
}
definition, err := loop.Define(
loop.WithName("assistant"),
loop.WithInference(client, selectedModel),
loop.WithRuntimeContext(gitContext{}),
loop.WithPolicyRevision("runtime-context-v2"),
)Because the provider is an opaque function-valued collaborator, configuring it
requires WithPolicyRevision. The revision is included in context request
fingerprints and the definition policy revision; a provider behavior change
must use a new revision. The provider is not copied into a durable event and
must be rebuilt by the composition root on restore.
Ownership and limits
Definition retains the provider interface; it does not assume ownership of
external resources the provider uses. The provider should honor ctx.Done()
and enforce its own byte/time bounds. Do not put credentials or unbounded file
contents in runtime blocks. If context counting is configured, the provider’s
revision is one of the inputs to loop.RequestFingerprint, so changing blocks
cannot masquerade as the same measured request.